A polyethylene glycol itself exhibits low toxicity and antigenicity and has excellent solubility toward water and many organic solvents. Therefore, end-activated polyethylene glycol compounds where a reactive functional group is introduced into an end thereof have been widely used for imparting various functions, for example, imparting stealthiness of drugs and drug carriers in the body and solubilizing them in drug delivery systems, and improving bio-compatibility of material surfaces. Of the compounds, a so-called hetero bifunctional polyethylene glycol having functional groups different in reactivity at two ends of a linear polyethylene glycol, respectively, can introduce different molecules, for example, bio-functional molecules such as drugs, physiologically active substances, targeting substances, and the like into respective ends. Accordingly, the polyethylene glycol is used as a hetero crosslinker which crosslinks these bio-functional molecules each other or the bio-functional molecules and various drug carriers or devices.
A first characteristic advantage obtained by using the hetero bifunctional polyethylene glycol as such a hetero crosslinker is (A) a fact that low antigenicity and excellent solubility, which are properties of a polyethylene glycol as described above, can be imparted to the bonded substance. Since a polyethylene glycol is a polymer having repeating units, it has considerable molecular weight and number of monomer units and the distance between the ends is long as compared with a usual low-molecular-weight hetero crosslinker. Therefore, the next advantage is (B) a fact that substances which are difficult to bond directly owing to steric hindrance or molecules or substances which may lose a function intrinsic to the molecules, such as pharmacological activity, by direct bonding, or substances having different natures, such as molecules or substances and various drug carriers or devices can be bonded each other.
In the field of DDS, there are many studies on conjugates using the hetero bifunctional polyethylene glycol.
Jiang et al. (Non-Patent Document 1) have bonded transferrin to β-lactoglobulin using a hetero bifunctional polyethylene glycol having an N-hydroxysuccinimidoester group and a maleimide group. Zhang et al. (Non-Patent Document 2) have bonded NLS (nuclear localization signal) that is a peptide decamer to a dendrimer bonded substance of acridine using a hetero bifunctional polyethylene glycol having an N-hydroxysuccinimidoester group and a maleimide group. Kim et al. (Non-Patent Document 3) have bonded siRNA to LHRH (luteinizing hormone releasing hormone) using a hetero bifunctional polyethylene glycol having an amino group or a 2-pyridyldisulfide group and a carboxylic acid. Anhorn et al. (Non-Patent Document 4) have bonded an IgG antibody to a nanoparticle including an anticancer agent using a hetero bifunctional polyethylene glycol having an N-hydroxysuccinimidoester group and a maleimide group.
Moreover, as an application to diagnostic devices, Otsuka et al. (Non-Patent Document 5) have showed utilization as a biosensor which monitors a bio-functional molecule such as lectin, where lactose is bonded to polylactic acid using a hetero bifunctional polyethylene glycol having an acetal group and a hydroxyl group and, further utilizing interaction between lactose and lectin and interaction between polylactic acid and an inorganic substance surface, as a result, lectin and the inorganic substance surface is crosslinked.
As above, a hetero bifunctional polyethylene glycol is a hetero crosslinker widely used in medicament-related uses such as modification of bio-functional molecules, especially in DDS filed. On the other hand, when it is considered to enhance the function of the polyethylene glycol bonded substance where the hetero bifunctional polyethylene glycol is reacted with various bio-functional molecules, carriers, and the like, there is a case where plural modification of either one bio-functional molecule is desired. For example, in the case where a polyethylene glycol bonded substance having a drug at one end and a targeting molecule at another end bonded thereto is considered, when plural bonding of the drug is achieved, it becomes possible to improve transport efficiency. Moreover, when plural bonding of the targeting molecule is achieved, it becomes possible to improve targeting performance toward a ligand.
As a polyethylene glycol compound taking such plural modification of the bio-functional molecule into consideration, there is a description of so-called Forked-PEG in U.S. Pat. No. 6,362,254 (Patent Document 1).
In the patent, though there is no application example to hetero polyethylene glycol derivatives, Forked-PEG where two functional groups are introduced into one polyethylene glycol end has been synthesized for creating a branching point at one end of a methoxy-polyethylene glycol and for modifying a bio-functional molecule.